196 LETTERS ON NATURAL MAGIC. 



but the one will gradually gain upon the other, 

 till at the fiftieth vibration it has gained half a 

 vibration on the other. At this instant the two 

 sounds will destroy one another, and an interval of 

 perfect silence will take place. The sound will 

 instantly commence, and gradually increase till 

 it becomes loudest at the hundredth vibration, 

 where the two vibrations conspire in producing a 

 sound double of either. An interval of silence 

 will again occur at the 150th, 250th, 350th vibra- 

 tion, or every second, while a sound of double 

 the strength of either will be heard at the 200th, 

 300th, and 400th vibration. When the unison 

 is very defective, or when there is a great differ- 

 ence between the number of vibrations which 

 the two strings or columns of air perform in a 

 second, the successive sounds and intervals of 

 silence resemble a rattle. With a powerful organ, 

 the effect of this experiment is very fine, the 

 repetition of the sounds wow wow wow re- 

 presenting the double sound and the interval of 

 silence which arise from the total extinction of 

 the two separate sounds. 



The phenomenon corresponding to this in the 

 case of light is perhaps still more surprising. If 

 a beam of red light issues from a luminous point, 

 and falls upon the retina, we shall see distinctly 

 the luminous object from which it proceeds ; but 

 if another pencil of red light issues from another 

 luminous point, anyhow situated, provided the 

 difference between its distance and that of the 

 other luminous point from the point of the retina, 

 on which the first beam fell, is the 258th thou- 

 sandth part of an inch, or exactly fam'ce, thrice, 

 four times, &c., that distance ; and if this second 

 beam falls upqn the same point of the retina, the 



